Unauthorized use of a telephone instrument can result in a substantial loss to a telephone subscriber. Such loss is compounded by the fact that unauthorized calls are difficult to detect if made surreptitiously, and may not become apparent until they are billed to the telephone subscriber.
There are known in the prior art many devices for preventing unauthorized use of a telephone instrument, particularly for dial-type telephones. The simplist device for dial-type telephones comprises a lock which secures to the dial, preventing rotation of the dial yet permitting the use of the telephone instrument for receiving incoming calls.
The advent of push button type telephones has made the task of preventing unauthorized calls more difficult, although there are devices in the prior art which are designed for such use. The following United States patents comprise the closest known prior art:
U.S. Pat. No. 3,866,000 PA1 3,899,647 PA1 3,965,410 PA1 4,005,279 PA1 4,081,630
Most of these patents disclose a security device which circumscribes a portion of the telephone instrument, and in particular includes a cover which prevents manual access to the push-button key pad of the telephone. However, the portions of these devices which surround the telephone make the instrument itself heavy and unwieldy. Furthermore, when the cover is opened to permit access to the key pad, the cover and its associated structure requires a great amount of space adjacent to the telephone itself. For these reasons, these prior art locking devices for push-button telephones have not gained wide acceptance. Other devices provide a cover which locks directly to the face plate of the key pad portion of a telephone. However, the face plate does not have the mechanical strength to resist tampering, so that the cover easily may be pried open to gain access to the key pad.